Sample preparation apparatus for direct numerical simulation of rock properties

ABSTRACT

A sample preparation apparatus and method of preparing a rock sample using such an apparatus, as useful in connection with the digital numerical simulation of properties of the rock. The disclosed apparatus includes a fixably mounted diamond wire cutter. Three linear translation stages are coupled to a specimen holder. One of the translation stages moves the specimen in a direction parallel to the plane of the cutting wire. The other two translation stages move the specimen in different directions from one another, and when actuated together, advance the specimen into the wire for short distances in a direction out of the plane of the cutting wire. Short piecewise linear cuts are made in the specimen, to provide a sample of the desired shape with a small cross-section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority, under 35 U.S.C. §119(e), ofProvisional Application No. 61/921,797, filed Dec. 30, 2013,incorporated herein by this reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

This invention is in the field of laboratory analysis of the physicalproperties of samples of material. Embodiments of this invention aredirected to an apparatus and method for obtaining a rock sample suitablefor high-resolution tomography and analysis via direct numericalsimulation.

Knowledge of the properties of the material of subsurface rockformations is important for assessing hydrocarbon reservoirs in theearth and formulating a development strategy regarding those reservoirs.Traditionally, samples of the rock formation of interest are subjectedto physical laboratory tests to determine these material properties,such properties also referred to as physical or petrophysicalproperties. However these tests are typically time consuming andexpensive. For example, the measurements of certain properties of aphysical rock sample require full water saturation of the sample, whichcan take an extremely long time if the rock has low permeability. Notonly are the results not available in a timely fashion, but these testsnecessarily occupy laboratory equipment over the duration of theexperiment, limiting the sample throughput and thus the number ofsamples that can be measured in a reasonable time. It is desirable toimprove the timeliness of analysis results and thus accelerate thedevelopment cycle, and also to increase the number of samples analyzedto improve the statistical confidence of the analysis results.

Direct numerical simulation of material properties from digital imagesof rock is a recent technology for determining the material propertiesof rock samples. According to this approach, an x-ray tomographic imageis taken of a rock sample to produce a digital image volumerepresentative of that sample. A computational experiment is thenapplied to the digital image volume to simulate the physical mechanismsfrom which the physical properties of the rock can be measured.Properties of the rock such as porosity, absolute permeability, relativepermeability, formation factor, elastic moduli, and the like can bedetermined using direct numerical simulation. In particular, directnumerical simulation is capable of estimating the material properties ofdifficult rock types, such as tight gas sands or carbonates, within atimeframe that is substantially shorter than that required for thecorresponding physical measurement. In addition, test equipment is notoccupied over long periods of time according to this technique, as theanalogous numerical conditions to the physical experiment can beimmediately applied by the computer simulation software.

The quality of the tomographic image of the rock sample is necessarily asignificant factor in the accuracy of the estimate of the materialproperties. X-ray tomography is based on the detection of differences inthe attenuation of the incident energy by the material components (e.g.,matrix space vs. pore space, or differences in rock composition). Toobtain accurate estimates of the material properties, it is importantthat these attenuation values accurately represent the structure andmaterial of the rock. Artifacts due to “beam hardening”, or thepreferential absorption of low energy photons in irregularly-shaped rocksamples, degrade the accuracy of the tomographic image. Morespecifically, beam hardening results from the mechanisms ofphotoabsorption, scattering, and photoelectric effect involved thatattenuate the X-rays. Because lower energy X-rays are more affected bythese mechanisms than are higher energy X-rays, the beam is said to“harden” in that the mean energy of the beam increases upon passingthrough the sample. The shape of the sample can cause this beamhardening to vary with position within the sample. If the cross-sectionof the sample is regularly shaped, for example circular, post-processingof the attenuation data readily compensates for these non-uniform beamhardening effects. However, if the sample has an irregular cross-sectionor otherwise has a variable thickness (e.g., polygonal cross-section),this post-processing is more difficult if not impossible. If beamhardening is not properly compensated, the digital image volume may notaccurately represent the material properties of the rock.

Another factor that affects tomographic image quality is the resolutionof the image, namely the size of the smallest detail distinguishable bythe imaging. Image resolution is controlled by characteristics of theacquisition system components and their spatial configuration relativeto the sample. Cross sectional sample size impacts image resolution, asthe minimum voxel size corresponds to the longest lateral dimension ofthe acquired image divided by the number of detector pixels representingthat longest lateral dimension. Samples in which the longest lateraldimension is relatively small (e.g., 2 mm) can thus be imaged at higherimage resolution, or smaller voxel size. It is also important for theimage volume “field of view” to be maximized so as to cover the largestpossible volume of rock under full illumination (i.e., the sampleremains in the field of view of the detector at all times).

Considering all of these factors, it has been observed that cylindricalsamples of rock of relatively small diameter (e.g., on the order of 2 to3 mm) provide the optimal cross-sectional shape and size for obtaininghigh quality tomographic images for direct numerical simulation usingmodern technology. These small cylindrical samples provide a regularlyshaped cross-section for which beam hardening is minimized andcorrectable, voxels of smaller size for improved resolution, and goodfield of view under full illumination.

In addition, the length of the cylindrical sample in the axial dimensionhas also proven to be important. It has been observed that the longestpossible axial extension of the sample maximizes the volume of materialthat is continuously imaged by a helical image acquisition system, andalso saves time in sample preparation and placement for standard(circular) image acquisition system geometries. The volume of materialthat is imaged should especially be maximized for the case ofcoarsely-grained and heterogeneous rock, to obtain an imaged volume thatis statistically representative of the formation from which the samplewas taken.

Considering these factors in combination, a cylindrical rock sample ofsmall cross-section (e.g., less than 3 mm) and relatively long axiallength (e.g., greater than 10 mm) is desirable for tomographic imagingfor direct numerical simulation, using conventional image acquisitionsystems. Meeting these geometrical requirements necessitates the cuttingof the sample that is to subsequently be imaged from a larger sample(e.g., a core sample, drill cuttings, etc.) that is itself obtained fromthe sub-surface formation of interest.

In addition to these geometric requirements, accurate direct numericalsimulation requires that the integrity of the material of the sampledformation be maintained in the sample to be imaged. More specifically,the preparation of the sample should not remove granular material fromthe edges of the sample volume, create fractures in the grains or matrixthat were not previously present, loosen grains at the sample perimeter,or otherwise deform grain shapes or pore space characteristics. Thisrequires cleanly, directly, and non-destructively cutting throughindividual grains of the rock.

Conventionally, the coring of a volume of rock to obtain a smallcylindrical sample suitable for imaging has been performed by drillingwith a hollow drill bit, commonly referred to as a “core bit”. It hasbeen observed that this coring technique is suitable for reliablyobtaining samples as small as 4 mm in diameter from some rock types. Atsmaller diameters, however, this approach tends to strip or fracturegrains of the rock, which destroys the sample. In addition, coring inthis manner has proven to be unsuitable for certain rock types,particularly rock that contains granular or sedimentary material that isnot highly consolidated.

Conventional core bits also are limited in the axial length of the thincylindrical sample that is obtained. Typically, the maximum axial lengthof a 3 mm core sample that can be obtained by a core bit is on the orderof 5 mm. As mentioned above, it is desirable to obtain samples forimaging that are significantly longer than 5 mm, especially for use inconnection with helical image acquisition systems.

Another conventional approach to the preparation of samples fortomographic imaging in the direct numerical simulation context is thecutting of rock with a diamond disc saw. This approach can obtainrelatively long samples of small cross-section along the axialdimension, with minimal degradation of the sample at its cut edges. Butbecause the disc saw is only able to cut along a two-dimensional plane,the prepared sample will have a rectangular cross-section, which resultsin significant loss of the imaged volume necessitated by compensationfor beam hardening, given the non-uniform distances traveled by theincident energy in the sample. For example, the resulting image volumefrom a parallelepiped sample contains only about 60% of the voxels thatcan be obtained from a similarly sized cylindrical sample. Otherdisadvantages resulting from the parallelepiped sample shape includepoor compatibility of the sample with flow or pressure cells, and theinability to perform “region of interest” (ROI) evaluations.

By way of further background, the preparation of samples for microscopyusing a diamond wire saw is known in the art. One example of aconventional diamond wire saw uses a thin stainless steel wire ontowhich industrial diamonds of varying grit size are embedded. The cuttingmotion can be either reciprocating or in one direction. Examples ofthese conventional diamond wire saws include those available from WellDiamond Wire Saws, Inc.

BRIEF SUMMARY OF THE INVENTION

Embodiments of this invention provide an apparatus and method forobtaining cylindrical samples of rock with extremely smallcross-sectional diameter for use in connection with tomographic imaging.

Embodiments of this invention provide such an apparatus and method thatare capable of obtaining such samples from various rock types withoutsignificantly degrading the material integrity of the sample.

Embodiments of this invention provide such an apparatus and method thatare capable of obtaining such samples from poorly consolidated rock,without requiring epoxy impregnation and similar techniques to maintainstructural integrity.

Embodiments of this invention provide such an apparatus and method thatis capable of obtaining samples of any one of a number ofcross-sectional shapes, including those of cylindrical, rectangular, andpolygonal cross-sections.

Other objects and advantages of embodiments of this invention will beapparent to those of ordinary skill in the art having reference to thefollowing specification together with its drawings.

Embodiments of the invention may be implemented into wire cutterapparatus, comprising a table, a wire supply drum and a guiding rollerthat are vertically displaced from the wire supply drum and that eachhave an axis parallel with one another, cutting wire wound about thewire supply drum and the guiding roller, and extending from the wiresupply drum around the guiding roller and back to the wire supply drumso that parallel lengths of the cutting wire extending between the wiresupply drum and the guiding roller define a cutting plane, a holder forholding a specimen of material to be cut by the cutting wire, and aplurality of translation stages movable relative to the table. Theplurality of translation stages comprise a feed translation stage,coupled to the holder, movable in a feed direction that is substantiallyparallel to the cutting plane; a first translation stage coupled to theholder that is movable in a first direction at an angle to the feeddirection, and a second translation stage coupled to the holder that ismovable in a second direction at an angle to the feed direction.

Embodiments of the invention may also be implemented into a method ofcutting a rock sample, comprising operating a wire saw to advance acutting wire from a wire supply drum around a guiding roller, whereforward and return lengths of the cutting wire run between the wiresupply drum and guiding roller and define a cutting plane, cutting apath from an edge of a specimen of rock to a starting point and thenactuating either or both of first and second translation stages tolinearly advance the specimen in a direction not parallel to the cuttingplane. After linearly advancing the specimen in the direction notparallel to the cutting plane, the method then involves stopping theadvancing of the specimen until the cutting wire substantiallystraightens, and repeating the actuating and stopping steps a pluralityof times to cut a closed figure in the specimen defining a perimeter ofthe sample. The specimen may then be withdrawn along the path.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an elevation view of a sample preparation apparatusconstructed according to an embodiment of the invention.

FIG. 2 is a perspective view of the translation stages and specimenholder of the apparatus of FIG. 1, according to that embodiment of theinvention.

FIG. 3 a plan view of the sample preparation apparatus constructedaccording to that embodiment of the invention.

FIG. 4 is a flow diagram illustrating a method of preparing a sampleaccording to an embodiment of the invention.

FIGS. 5a through 5e are schematic views of a specimen and the cuttingwire subsystem of the apparatus of FIG. 1, at various stages of themethod of FIG. 4, according to that embodiment of the invention.

FIG. 5f is a schematic plan view of the perimeter of a sample cut from alarger specimen, at a stage of the method of FIG. 4 according to thatembodiment of the invention.

FIG. 6 is a flow diagram illustrating a method of analysis of a rocksample according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be described in connection with its embodiments,namely as implemented into an apparatus and method of preparing a rocksample for use in digital numerical simulation analysis of theproperties of the rock from which that sample was acquired, as it iscontemplated that this invention will be especially beneficial in suchan application. However, it is contemplated that this invention will beuseful and beneficial in other applications beyond those described inthis specification. Accordingly, it is to be understood that thefollowing description is provided by way of example only, and is notintended to limit the true scope of this invention as claimed.

As discussed above in connection with the Background of the Invention,embodiments of this invention pertain to the acquiring of rock samplesand their analysis by way of direct numerical simulation. As such, it iscontemplated that embodiments of this invention will be especiallybeneficial in the acquiring of rock samples from sub-surface formationsimportant in the exploration and production of oil and gas. Morespecifically, the rock from which these samples will be acquired arecontemplated to correspond to formations accessed by terrestrial ormarine drilling systems such as used to extract resources such ashydrocarbons (oil, natural gas, etc.), water, and the like from thoseformations. As is fundamental in the art, the optimization of oil andgas production operations is largely influenced by the structure andphysical properties of these sub-surface rock formations. The samplesobtained according to embodiments of this invention are useful inunderstanding those formation attributes.

As will be evident from the following description, embodiments of thisinvention are more specifically directed to the obtaining of smallsamples of rock from larger samples of the rock of interest that werepreviously recovered from the sub-surface. For the sake of clarity,those larger samples of rock will be referred to as “specimens” in thisdescription, and the small samples of rock obtained from those specimenswill be referred to as “samples”. No particular connotation is intendedby the separate terms “specimens” and “samples”; rather, the use ofthese separate terms is merely intended to distinguish the small samplesobtained according to this invention from the larger samples from whichthose small samples are obtained.

Conventional diamond wire cutters are used for the preparation ofsamples, including samples of rock that are obtained for petrophysicalproperty analysis. An example of such a conventional diamond wire cutteris the 3242 Diamond Wire Cutter available from Well Diamond Wire Saws,Inc. However, it has been observed, in connection with this invention,that conventional wire cutters such as that 3242 Diamond Wire Cutter arenot readily capable of cutting in a curved path as is necessary for thepreparation of cylindrical samples, particularly those with a small (<10mm) cross-sectional diameter. One reason for this limitation is that thearrangement of the cutting wire in these conventional cutters, generallyrunning vertically between a wire supply drum and a guiding roller, donot allow shear forces to be applied to the cutting wire. In suchconventional wire cutters, these shear forces can cause the cutting wireto become stuck in the workpiece, or to become dislodged from theguiding roller. In addition, because the diamond cutting wire bendsduring cutting, with a curvature corresponding to the applied force andalso the working length of the workpiece material, an even cut can onlybe obtained in one direction, namely the feed direction. Theseconventional diamond wire cutters such as the 3242 Diamond Wire Cutteralso provide only a single degree of freedom in the movement of thecutting wire relative to the workpiece. For example, the 3242 DiamondWire Cutter maintains a fixed position of the workpiece, with the onlypermitted movement being the movement of the cutting wire toward andaway from the workpiece. This single degree of freedom provided by thisconventional diamond wire cutter necessitates a unidirectional cut.

FIG. 1 illustrates the construction of sample preparation apparatus 10according to an embodiment of this invention. As will become apparentfrom the following description, sample preparation apparatus 10 iscapable of obtaining small samples of the desired cross-section,typically circular but also other shapes such as polygons, from a largerspecimen of the rock of interest. It is contemplated that the manner inwhich the rock specimens are obtained from the sub-surface, and thephysical form of those specimens, can vary widely. Examples of rockspecimens useful in connection with embodiments of this inventioninclude whole core samples, side wall core samples, outcrop samples,drill cuttings, and laboratory generated synthetic rock samples such assand packs and cemented packs.

In this embodiment of the invention shown in FIG. 1, sample preparationapparatus 10 includes table 11, which provides a stable base for theother components of apparatus 10. Wire supply drum 12 is mounted totable 11 via its motor enclosure 11M, and is motor-driven to rotateabout its axis. In this embodiment, guiding roller 14 is mounted totable 11 at a vertical position below wire supply drum 12, with its axisparallel to that of wire supply drum 12. Diamond cutting wire 13 iswound about wire supply drum 12, around guiding roller 14, and back towire supply drum 12 as shown. The vertical position of guiding roller 14relative to wire supply drum 12 may be adjustable, providing atensioning device to maintain the desired tautness in cutting wire 13.Cutting wire 13 of conventional construction, for example adiamond-impregnated wire of about 60 μm grit and about 300 μm indiameter, is suitable for sample preparation of typical rocks ofinterest; of course the grit and diameter of cutting wire can varyaccording to the sample materials.

Table 11, wire supply drum 12, and guiding roller 14 in apparatus 10 aresimilar components as provided in conventional diamond wire cutters,such as the 3242 Diamond Wire Cutter. According to embodiments of thisinvention, motor enclosure 11M may be unitary with table 11, oralternatively may be a separate module that is attached to table 11. Inthe conventional operation of the 3242 Diamond Wire Cutter, its motorenclosure moves, relative to its table, to advance the cutting wire tothe specimen. However, according to embodiments of this invention, ifmotor enclosure 11M is a separate module, motor enclosure 11M is mountedto table 11 in a fixed position by cap screws 23 and bracket 25 as shownin FIG. 1; another pair of cap screws 23 and another bracket 25 are alsopresent on the other side of motor enclosure 11M in that FIG. 1. Becausemotor enclosure 11M is in a fixed position relative to table 11, wiresupply drum 12 and guiding roller 14, and thus cutting wire 13, are alsoin a fixed position relative to table 11.

Conversely, according to embodiments of this invention, apparatus 10 isconstructed so that the workpiece, namely specimen 15 of FIG. 1 fromwhich the rock sample is to be cut, is movable in multiple degrees offreedom relative to cutting wire 13, which is maintained at a fixedposition. In apparatus 10 shown in FIG. 1, the position of positioningtable 21 on table 11 can be adjusted along tracks in positioning table21, and then fixed in position by fixing screws 22. In any case, it iscontemplated that, once adjusted and set, positioning table 21 willremain fixed in position relative to table 11 during sample preparation.

According to this embodiment of the invention, three linear translationstages 18, 19, 20 are coupled to positioning plate 21. Morespecifically, linear translation stages 18, 19, 20 in this example aremodular positioning stages, mounted in a stacked manner relative to oneanother. As shown in FIG. 1 generally, and in more detail in FIG. 2,y-translation stage 18 is mounted to positioning table 21, x-translationstage 19 is mounted to y-translation stage 18, and f-translation stage20 is mounted to x-translation stage 19.

Translation stages 18, 19, 20 in this embodiment of the invention areconventional linear translation stages as known in the art. For example,each of translation stages 18, 19, 20 may be constructed to have a stagebody that is mountable (e.g., by way of bolts) to a fixed plate, and acarriage that is movable in a single direction along a track or railunder the control of an actuator. FIGS. 2 and 3 illustrate micrometeractuators 18 a, 19 a, 20 a coupled to translation stages 18, 19, 20,respectively. The directions of travel provided by each of translationsstages 18, 19, 20 is indicated in FIGS. 2 and 3 by the “y”, “x”, and “f”arrows, respectively. An example of a suitable linear translation stagesuitable for use as translation stages 18, 19, 20 is the M-UMR 8.51manual translation stage, with the BM17.51 micrometer actuator, asavailable from Newport Spectra-Physics, Ltd.

For the example of modular translation stages 18, 19, 20 as shown inFIGS. 1 and 2, and as mentioned above, the stage body of y-translationstage 18 is fixed by way of bolts or the like to positioning plate 21,such that movement of its carriage relative to its stage body, and thusrelative to positioning plate 21, will be along the y-direction as shownin FIG. 2. In this example, the stage body of x-translation stage 19 isfixed by way of bolts or the like to the carriage of y-translation stage18, such that movement of its carriage relative to its stage body, andthus relative to the carriage of y-translation stage 18, will be alongthe x-direction shown in FIG. 2. In this example, L-shaped plate 26 ismounted to the carriage of x-translation stage 19, and the stage body off-translation stage 20 is mounted to plate 26 by way of bolts 27, suchthat f-translation stage 20 is mounted perpendicularly to translationstages 18, 19. Movement of the carriage of f-translation stage 20relative to its stage body, and thus relative to the carriage ofx-translation stage 19, will be along the f-direction shown in FIG. 2.

In this embodiment, as described above and as will be described below,guiding roller 14 is vertically displaced relative to wire supply drum12, so that the paths of cutting wire 13 between wire supply drum 12 andguiding roller 14 are substantially vertical; in this arrangement, thedirections of travel of x-translation stage 19, y-translation stage 18,and f-translation stage 20 are all in a horizontal plane that isorthogonal to the cutting plane defined by the vertical paths of cuttingwire 13. It is contemplated, however, that guiding roller 14 may bedisplaced in a direction other than vertical relative to wire supplydrum 12. For example, guiding roller 14 may be mounted so that the pathsof cutting wire 13 travel in a horizontal path. In that case,x-translation stage 19, y-translation stage 18, and f-translation stage20 would be rotated accordingly, so that their respective directions oftravel would be in a vertical plane that is orthogonal to the horizontalplane. Of course, orientations other than the vertical and horizontalare also contemplated in connection with this embodiment.

It is contemplated that other types of translation stages, includingintegrated translation stages replacing two or more of modulartranslation stages 18, 19, 20 may alternatively be used. Alternatively,one or more of translation stages 18, 19, 20 may be provided with amotorized actuator instead of the micrometer actuators 18 a, 20 a asshown. According to this alternative approach in which actuators 18 a,19 a, 20 a are motorized, it is contemplated that sample preparationapparatus 10 may also include, if desired, a computer or otherprogrammable controller capable of controlling these actuators 18 a, 19a, 20 a according to a pre-programmed sequence, so as to automate thecutting of a sample from specimen 15 in a consistent and repeatablemanner. In this embodiment, x-translation stage 19 and y-translationstage 18 are orthogonal to one another, and as such have carriages thatare movable in directions that are substantially perpendicular to oneanother, as it is believed that such an arrangement will facilitateefficient control, it is further contemplated that these translationstages may alternatively be oriented at an angle other thanperpendicular to one another if desired.

In this embodiment of the invention, specimen 15 (shown in FIG. 1) isheld by specimen holder 16, which is mounted to f-translation stage 20by way of vertical adjustment plate 17 and bracket arrangement 28.Specimen holder 16 is contemplated to include jaws or another type ofclamping arrangement for securely holding specimen 15 during the cuttingprocess. Vertical adjustment plate 17 allows adjustment of the verticalposition of specimen holder 16, and thus specimen 15. In this embodimentof the invention, the fixed coupling of specimen holder 16 tof-translation stage 20, and the stacked arrangement of translation stags18, 19, 20, allows a translation by one or more of translation stages18, 19, 20 to effect movement of specimen 15 in the correspondingdirections.

As shown in the plan view of FIG. 3 in combination with FIGS. 1 and 2,the f-direction of translation by f-translation stage 20 is contemplatedto be substantially parallel to a cutting plane defined by the two pathsfollowed by cutting wire 13 that extends from wire supply drum 12 aroundguiding roller 14 and back. According to embodiments of the invention,as will be described in further detail below, translation of specimen 15along the f-direction is used for advancing specimen 15 toward wire 13,and for withdrawing specimen 15 from wire 13. During the remainder ofthe cutting process, particularly in the cutting of a closed figure todefine the sample being cut from specimen 15, translation of specimen 15will be controlled by x-translation stage 19 and y-translation stage 18.

According to this embodiment of the invention, apparatus 10 providesdegrees of freedom, in the x and y directions, that enable the cuttingof samples of varying and arbitrary cross-sectional shape from rockspecimens. And as will be described in detail below, the operation ofapparatus 10 according to embodiments of the invention enable thecutting of samples of very small cross-sectional diameter, thus reducingvoxel size within the imaged volume, which improves the accuracy of thematerial property estimates derived via direct numerical simulation.

Referring now to FIG. 4 in combination with the schematic diagrams ofFIGS. 5a through 5e , the operation of apparatus 10 in preparing asample from a specimen of rock according to embodiments of thisinvention will now be described. As described above, specimen 15 may beacquired in any one of a number of conventional ways. In the context ofthe oil and gas industry, specimen 15 will typically be derived from thedrilling of exploration or production wells, and as such may come fromwhole core samples, side wall core samples, outcrop samples, and drillcuttings; alternatively, specimen 15 may be produced from a laboratorygenerated synthetic rock sample such as a sand pack or a cemented pack.According to embodiments of this invention, the nature of the rock fromwhich specimen 15 consists can be quite wide ranging, including lessconsolidated and structurally robust materials such as sandstones,clays, and other granular or sedimentary material that is not highlyconsolidated.

Sample preparation process 200 begins, in this embodiment of theinvention, with process 30 in which specimen 15 is placed into andretained by specimen holder 16. For the example in which specimen holder16 includes a pair of jaws, process 30 consists of the fixing ofspecimen 15 in those jaws. In many cases, a cylindrical sample isdesired to be cut from specimen 15, in which case it is desirable forspecimen 15 to have flat top and bottom surfaces, and have a thicknesscorresponding to the desired length of the sample to be recovered. Asshown in FIG. 5a , specimen 15 may have a cylindrical shape (i.e.,disk-shaped), as typical for prepared core samples obtained from thedrilling process.

Once placed in specimen holder 16, sample 15 is positioned and orientedat the desired location of cutting wire 13 in process 32, as shown inFIG. 5a . The vertical position of specimen 15 is adjusted by way ofvertical adjustment plate 17. For the case of a disk-shaped specimen 15,its flat surfaces will be optimally oriented to be perpendicular tocutting wire 13, to produce a cylindrical sample. In process 34,f-translation stage 20 is aligned so that its movement will be parallelto the cutting plane defined by cutting wire 13. Referring to FIG. 5a ,cutting wire 13 is shown as having two vertical lengths 13 d, 13 u,extending from wire supply drum 12 around guiding roller 14 verticallydisplaced beneath wire supply drum 12, and back again, with the verticallengths 13 d, 13 u being substantially parallel to one another, defininga plane referred to in this description as the cutting plane. In thisexample, motor 11M operates so that the cutting motion of cutting wire13 is reciprocal, as is typical for wire saws. Alternatively, roller 14may be replaced by a second wire supply drum if desired. In the exampleof apparatus 10 described above, the alignment of f-translation stage 20in process 34 is contemplated to be accomplished by moving positioningplate 21 (to which translation stages 18, 19, 20 are mounted) relativeto table 11, and fixing positioning plate 21 in place by tighteningfixing screws 22. Proper alignment of f-translation stage 20 so thattranslation is parallel to the cutting plane ensures that specimen 15may be cut into for the desired length without imparting shear forces onwire length 13 d. Alignment process 34 optimally places specimen 15 asclose to wire length 13 d as practicable, so that much of the travellimit of f-translation stage 20 will be within specimen 15; it is alsouseful for actuators 18 a, 19 a of translation stages 18, 19,respectively, to be initially set at their medial values so that eachcan exert the maximum travel in either direction.

In process 36, f-translation stage 20 is actuated (via actuator 20 a) toadvance specimen 15 toward and against cutting wire length 13 d. Thistranslation of specimen 15, in the f-direction only, bends wire length13 d as shown in FIG. 5b , but this bending is in the cutting planedefined by wire lengths 13 d, 13 u, and as such imparts minimal shearforces on cutting wire 13. As such, the cutting of process 36 may beperformed “non-stop” if desired. In any case, process 36 continues untila path of the desired length is cut into specimen 15. More specifically,it is contemplated that this path will extend from the perimeter ofspecimen 15 up to the point within specimen 15 at which the perimeter ofthe sample to be cut will begin.

Following process 36, the cutting of the perimeter of a sample fromspecimen 15 begins in process 38, with the actuation of one or both ofactuators 18 a, 19 a to move either or both of x- and y-translationstages 18, 19, respectively, and thus specimen 15 for a short distanceaccording to the desired sample perimeter. According to embodiments ofthis invention, the resulting translation by either or both of x- andy-translation stages 18, 19 will generally be out of the f-directioncutting plane defined by wire lengths 13 d, 13 u, and as such shearforces will be applied against wire length 13 d. However, the effect ofthese shear forces is minimized by limiting the distance and rate atwhich specimen 15 is moved in process 38. For example, the distance ofthe translation in process 38 is very short, for example no more thanabout 100 μm for the example of apparatus 10 based on the Model 3242Diamond Wire Cutter referenced above. The feed rate of cutting wire 13from wire supply drum 12 will depend on a number of factors, includingthe composition of specimen 15, the thickness of cutting wire 13, thetranslation distance for each movement of specimen 15, and the like. Forexample, cutting wire 13 of a diameter of about 100 μm may be fed at arate up to about 50 μm/sec, to cut a sandstone specimen 15. Thickercutting wire 13 may allow a higher maximum feed rate. In any case, it iscontemplated that those skilled in the art having reference to thisspecification will be readily able to determine a suitable feed rate andcutting wire type and diameter. These constraints of maximum translationdistance and maximum wire feed rate will limit the curvature of wirelength 13 d from the vertical, and thus limit the shear forces.

After the short translation of process 38, movement of specimen 15 isceased in process 40 for at least a minimum length of time to allowcutting wire length 13 d to return to a straight orientation. Duringthis wait time of process 40, cutting wire length 13 d acts to removematerial from specimen 15 along the length of the translation of process38, straightening out as it does so, which in this arrangement resultsin cutting wire length 13 d returning to the vertical. It iscontemplated that the wait time of process 40 for cutting wire length 13d to become substantially straight will be on the order of at leastabout 3 seconds ranging up to about 5 seconds, for most rock materialsof interest in the oil and gas context. If specimen 15 has beenimpregnated with epoxy to reduce damage, as is conventionally done formedium to poorly consolidated samples, this wait time may be muchlonger, for example as long as several minutes. It has been observedthat waiting process 40 is not only beneficial to maintain the health ofcutting wire 13, but also results in a straight cut throughout the depthof specimen 15, and thus good control the shape of the sample that willeventually be removed. At the end of this wait time, the combination oftranslation process 38 and wait process 40 will have resulted in thecutting of a short linear distance in specimen 15.

In decision 41, the user determines whether the perimeter of the samplebeing cut from specimen 15 is complete, in that the most recent linearcut has completed a closed figure within specimen 15. If not (decision41 is “no”), process 38 is repeated by the actuation of either or bothof x-translation stage 18 and y-translation stage 19 for a shortdistance. For the cutting of an approximated circle in specimen 15, thedirection of each successive translation process 48 will be in adifferent direction from the previous. Alternatively, apparatus 10 andits operation according to embodiments of the invention can also be usedto cut a polygonal cross-section, in which case the translation of anext instance of process 38 may be in the same direction as theprevious. Waiting process 40 is then performed again to allow wirelength 13 d to make the cut and remove the material, straightening so asto return to the vertical. These processes 38, 40 are then repeateduntil it is determined, in decision 41, that the full perimeter of thesample has been cut.

FIG. 5c schematically illustrates specimen 15 after a number ofpiecewise linear cuts resulting from repeated processes 38, 40,according to an embodiment of the invention. At the stage of the processshown in FIG. 5c , a portion of a circular cross-section has beendefined by cutting wire 13. FIG. 5f illustrates, in plan view, thispartial cutting of the sample in further detail. As shown in FIG. 5f ,path 50 was cut, in process 36, from the outer surface of specimen 15 topoint 51. The small linear cuts of processes 38, 40 began from point 51with cut 52 ₁, and repeated to form cuts 52 ₂, 52 ₃, and so on in acounter-clockwise direction in this example. (The endpoints of each cut52 are emphasized for purposes of this explanation, but will not in factbe present in specimen 15.)

As shown in FIG. 4, optional adhesive process 42 may be performed, ifdesired, at one or more points during the repeated linear cuts formed byprocesses 38, 40. In process 42, an adhesive is applied along part ofthe perimeter of the sample already cut, for example after aboutthree-fourths of the perimeter has been cut, to keep the sample fromfalling out upon completion of the cut. The presence of this adhesiveapplied in process 42 also ensures that the sample is fully cut fromspecimen 15, rather than prematurely breaking off from specimen 15 asthe perimeter cut is nearing completion.

According to embodiments of this invention, the repeated processes 38,40 continue to form cuts 52 in the same manner until forming a closedfigure upon returning to point 51, as determined by decision 41. Thisstage of the process is illustrated in FIG. 5d , with sample S having aperimeter defined by the closed figure formed by the sequence of linearcuts 52. In this example, because each of cuts 52 is quite short, forexample no longer than about 100 μm, the sequence of cuts 52 is a goodapproximation of circle 54, which yields a cylindrical sample fromdisk-shaped specimen 15. For example, it is contemplated that on theorder of 60 cuts of 100 μm will cut a circle of about 2 mm in diameter,which is very useful in the context of tomography and digital numericalsimulation.

Upon completion of the repeated linear cuts by processes 38, 40(decision 41 returning a “yes” result), translation in the x- andy-directions ceases. In the example of FIG. 5f , wire length 13 d is atpoint 51 at this point. Process 44 is then performed to withdrawspecimen 15 (with sample S) from cutting wire 13, by actuatingf-translation stage 20 in the direction parallel to the cutting plane(in the opposite direction from that of process 36). It is contemplatedthat little or no additional cutting of specimen 15 will typically takeplace in process 44.

Following process 44, sample S is then removed from specimen 15, forexample by removing the adhesive applied in process 42 if present, or byotherwise pushing sample S from specimen 15, in process 46. FIG. 5eschematically illustrates the removal of sample S from specimen 15,following withdrawal of specimen 15 from cutting wire length 13 d inprocess 44. Specimen 15 may be removed from specimen holder 16 eitherprior to, or after, removal process 46. Alternatively, specimen 15 maybe repositioned in specimen holder 16 (with sample S retained inspecimen 15 by adhesive, if desired), and sample preparation process 200repeated, if another sample is to be cut from this same specimen 15.

According to the embodiment of the invention described above, in whichspecimen 15 is advanced by f-translation stage 20 in process 36 in thef-direction parallel to the cutting plane of wire lengths 13 d, 13 u,the overall cutting time can be minimized as the cutting of specimen 15from its edge to starting point 51 can be done continuously, withoutstopping. Alternatively, specimen 15 may be cut from its edge tostarting point 51 along a path running in directions that are notparallel to the cutting plane, by way of a series of short piecewiselinear cuts carried out by translations in the x- and y-directions bytranslation stages 18, 19, separated by wait times, such as performed inprocesses 38 and 40 to cut the perimeter of the sample. Thisnon-parallel approach may be useful for particular sample geometries, orif specific portions of specimen 15 are to be avoided.

As mentioned above, sample preparation apparatus 10 may be constructedto include a computer or other programmable controller that controls thesequence in which actuators 18 a, 19 a, 20 a operate to move specimen15. This automated approach to sample preparation can be particularlyuseful in ensuring that the appropriate wait time following one of thepiecewise cuts elapses before initiating the next cut. In connectionwith this automated implementation, it is further contemplated thatsensors may also be implemented into sample preparation apparatus 10,for example to sense the time at which wire 13 d returns to the verticalfollowing translation of specimen 15, following which the translation ofspecimen 15 in the direction of a next cut can then begin.

Referring now to FIG. 6, the overall process of the estimation ofmaterial properties using digital numerical simulation, from samplesprepared according to embodiments of this invention, will be described.The estimation process begins with sample preparation process 200,carried out in the manner described above relative to FIGS. 4 and 5 athrough 5 f according to embodiments of this invention to produce one ormore rock samples for imaging.

In process 202, an imaging system obtains two-dimensional (2D) orthree-dimensional (3D) images, or other appropriate imagerepresentations, of the rock sample prepared in process 200. Theseimages and representations obtained in process 202 include details ofthe internal structure of the samples. An example of the imaging deviceused in process 202 is an X-ray computed tomography (CT) scanner, of atype, construction, or other attributes corresponding to any one of anumber of x-ray devices capable of producing an image representative ofthe internal structure of the sample of the desired resolution. Forexample, a plurality of two-dimensional (2D) sectional images of thesample may be acquired, and forwarded to a computing device that thenconstructs a three-dimensional (3D) digital image volume correspondingto the sample. Conventional computing devices suitable for performingthis construction and the subsequent analysis may be any one of a numberof conventional computers, for example, a desktop computer orworkstation, a laptop computer, a server computer, a tablet computer,and the like, having sufficient computational capacity to carry out thedesired operations.

Specific conventional techniques for acquiring and processing 3D digitalimage volumes of the sample in process 202 include, without limitation,X-ray tomography, X-ray micro-tomography, X-ray nano-tomography, FocusedIon Beam Scanning Electron Microscopy, and Nuclear Magnetic Resonance.

This image volume is typically represented by 3D regular elements calledvolume elements, or more commonly “voxels”, each having an associatednumeric value, or amplitude, that represents the relative materialproperties of the imaged sample at that location of the representedmedium. In process 210, the computing device performs segmentation orother image enhancement techniques on the digital image volume of thesample to distinguish and label different components in the imagevolume. For example, segmentation process 210 may identify thesignificant elastic components, such as pore space and mineralogicalcomponents (e.g., clays and quartz), that can affect the elasticcharacteristics of the sample. Segmentation process 210 may be performedto identify more than two significant elastic phases, representing suchmaterial constituents as pore space, clay fraction, grain contacts, andindividual grains and minerals. The particular segmentation algorithmused by the computing device in process 210 may vary according to theanalysis desired; typically some type of “thresholding” is applied, togroup voxels having similar amplitudes with one another. Conventionalimage processing to enhance the image volume, to reduce noise, etc. maybe included in process 210 as known in the art.

In process 220, a computing device then performs digital numericalsimulation to analyze one or more physical properties of the sample,typically by way of numerical analysis of the thresholded digital imagevolume. The properties that may be determined in process 220 includebulk elastic properties of the rock. In the context of oil and gasexploration and production, petrophysical properties of interest such asporosity, permeability, formation factor, permeability, relativepermeability, electrical conductivity, mercury capillary injection, andthe like, may be determined in process 220. These petrophysicalproperties may be estimated using an appropriate discretization or meshof the evolved pore space, combined with appropriate numericalsimulation, e.g. the direct numerical simulation of single phase fluidflow for computation of absolute permeability. The determination of someof these petrophysical properties in process 220 may also requirenumerical simulation using finite element methods, finite differencemethods, finite volume methods, Lattice Boltzmann methods or any varietyof other numerical approaches.

The method of preparing rock samples, and the apparatus for carrying outsuch preparation, according to embodiments of this invention providesimportant benefits and advantages, particularly for samples to besubjected to X-ray tomography for direct numerical simulation.Embodiments of this invention enable the preparation of cylindrical rocksamples with extremely small cross-sectional diameters, for examplediameters of 3 mm or less, which allow extremely high resolutiontomographic imaging as is necessary to resolve fine structural detail.These samples can be obtained from a wide range of various rock types,including poorly consolidated or otherwise fragile rock, withoutsignificantly degradation of material integrity or pore structure. Inaddition, embodiments of the invention provide flexibility in the samplepreparation process, enabling the cutting of samples having any one of anumber of cross-sectional shapes, including those of circular,rectangular, and polygonal cross-sections.

While this invention has been described according to one or more of itsembodiments, it is of course contemplated that modifications of, andalternatives to, these embodiments, such modifications and alternativesobtaining the advantages and benefits of this invention, will beapparent to those of ordinary skill in the art having reference to thisspecification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

What is claimed is:
 1. A wire cutter apparatus, comprising: a table; awire supply drum; a guiding roller vertically displaced from the wiresupply drum, the guiding roller and the wire supply drum each having anaxis parallel with one another; cutting wire, wound about the wiresupply drum and the guiding roller, and extending from the wire supplydrum around the guiding roller and back to the wire supply drum, so thatparallel lengths of the cutting wire between the wire supply drum andthe guiding roller define a cutting plane; a holder for holding aspecimen of material to be cut by the cutting wire; and a plurality oftranslation stages movable relative to the table, comprising: a feedtranslation stage, coupled to the holder, movable in a feed directionthat is substantially parallel to the cutting plane; a first translationstage, coupled to the holder, movable in a first direction at an angleto the feed direction; and a second translation stage, coupled to theholder, movable in a second direction at an angle to the feed direction;wherein the feed direction, the first direction, and the seconddirection are coplanar with one another.
 2. The apparatus of claim 1,wherein the first and second directions are substantially perpendicularto one another.
 3. The apparatus of claim 1, wherein the wire supplydrum and guiding roller are mounted at a fixed position relative to thetable.
 4. The apparatus of claim 1, further comprising: a positioningplate fixably mounted to the table; wherein each of the feed translationstage and the first and second translation stages are movable relativeto the positioning plate.
 5. The apparatus of claim 4, wherein theholder comprises: jaws for holding the sample; and a support coupled tothe jaws, and coupled to the feed translation stage so as to be movablewith the feed translation stage relative to the positioning plate. 6.The apparatus of claim 1, wherein the plane of the feed direction, firstdirection, and second direction is orthogonal to the cutting plane. 7.The apparatus of claim 6, wherein the cutting plane is orientedvertically, and the plane of the feed direction, first direction, andsecond direction is oriented horizontally.
 8. The apparatus of claim 6,wherein the cutting plane is oriented horizontally, and the plane of thefeed direction, first direction, and second direction is orientedvertically.
 9. A wire cutter apparatus, comprising: a table; a wiresupply drum; a guiding roller vertically displaced from the wire supplydrum, the guiding roller and the wire supply drum each having an axisparallel with one another; cutting wire, wound about the wire supplydrum and the guiding roller, and extending from the wire supply drumaround the guiding roller and back to the wire supply drum, so thatparallel lengths of the cutting wire between the wire supply drum andthe guiding roller define a cutting plane; a holder for holding aspecimen of material to be cut by the cutting wire, comprising: jaws forholding the sample; a support coupled to the jaws, and coupled to thefeed translation stage so as to be movable with the feed translationstage relative to the positioning plate; and a vertical adjustmentplate, for adjusting the vertical position of the jaws relative to thepositioning plate; a plurality of translation stages movable relative tothe table, comprising: a feed translation stage, coupled to the holder,movable in a feed direction that is substantially parallel to thecutting plane; a first translation stage, coupled to the holder, movablein a first direction at an angle to the feed direction; and a secondtranslation stage, coupled to the holder, movable in a second directionat an angle to the feed direction; and a positioning plate fixablymounted to the table; wherein each of the feed translation stage and thefirst and second translation stages are movable relative to thepositioning plate.
 10. A wire cutter apparatus, comprising: a table; awire supply drum; a guiding roller vertically displaced from the wiresupply drum, the guiding roller and the wire supply drum each having anaxis parallel with one another; cutting wire, wound about the wiresupply drum and the guiding roller, and extending from the wire supplydrum around the guiding roller and back to the wire supply drum, so thatparallel lengths of the cutting wire between the wire supply drum andthe guiding roller define a cutting plane; a holder for holding aspecimen of material to be cut by the cutting wire, comprising: jaws forholding the sample; and a support coupled to the jaws, and coupled tothe feed translation stage so as to be movable with the feed translationstage relative to the positioning plate; a plurality of translationstages movable relative to the table, comprising: a feed translationstage, coupled to the holder, movable in a feed direction that issubstantially parallel to the cutting plane, and comprising: a lineartranslation stage coupled to the support of the holder and to thepositioning table; and an actuator, coupled to the linear translationstage, for controlling movement of the linear translation stage alongthe feed direction; a first translation stage, coupled to the holder,movable in a first direction at an angle to the feed direction; and asecond translation stage, coupled to the holder, movable in a seconddirection at an angle to the feed direction; and a positioning platefixably mounted to the table; wherein each of the feed translation stageand the first and second translation stages are movable relative to thepositioning plate; and wherein each of the first and second translationstages comprises: a linear translation stage coupled to the positioningtable; and an actuator for controlling movement of the lineartranslation stage.
 11. The apparatus of claim 10, wherein each of theactuators comprise a micrometer actuator.
 12. The apparatus of claim 10,wherein each of the actuators comprise a motorized actuator.
 13. Theapparatus of claim 12, further comprising: a programmable controller,coupled to each of the actuators, programmed to control the actuators tomove the translation stages according to a pre-programmed sequence. 14.The apparatus of claim 10, wherein the feed translation stage and thefirst and second translation stages each comprise a modular translationstage; and wherein the first translation stage is mounted to thepositioning plate, the second translation stage is mounted to the firsttranslation stage, and the feed translation stage is mounted to thesecond translation stage.
 15. The apparatus of claim 10, wherein two ormore of the feed translation stage and the first and second translationstages comprise an integrated translation stage.